Combined steam turbine gas turbine cycle



June 20, 1967 TEMPERATURE F R. c. SHELDON 3,325,992

COMBINED STEAM TURBINE GAS TURBINE CYCLE FUEL Filed April 26, 1966 FIG! 5o HEAT INVENTOR RICHARD c. SHELDON, BY W HIS ATTORNEY.

United States Patent 3,325,992 COMBKNED STEAM TURBINE GAS TURBINE CYCLE Richard C. Sheldon, Schenectady, N.Y., assignor to General Electric Qompany, a corporation of New York Filed Apr. 26, 1966, Ser. No. 545,356 4 Claims. (Cl. 6039.18)

ABSTRACT 0F THE DISCLOSURE Combined steam turbine gas turbine cycle employs HP and LP gas turbine sections exhuasting through HP and LP heat exchangers respectively containing paralleled steam evaporators supplying common saturated steam drum.

This invention relates to an improved combined steam turbine gas turbine cycle of the type Where steam is obtained for the steam turbine from unfired boilers receiving their heat from the gas turbine exhaust.

It is known that steam can be generated by recovering the waste heat in a gas turbine in order to operate a steam turbine in a combined cycle. The heat is usually recovered from the lowest pressure gas turbine section which discharges gas through banks of heat exchange tubes at a pressure corresponding to the gas turbine discharge pressure, where it is reduced in temperature while generating steam in the tubes. The tube banks can also include sections for feedwater heating and superheating of the steam in a known manner.

One of the disadvantages of a simple atmospheric pressure heat recovery unit is one of cost in adding on the heat recovery unit, due to the relatively low heat transfer coefficients obtained at atmospheric pressure. Therefore, the size and cost of the heat recovery boiler soon offsets the savings to be made through the use of a combined cycle.

Various suggestions have also been made regarding the obtaining of more eflicient heat transfer by means of partially or Wholly carrying out the combustion process in a steam generation unit using supplementary firing of fuel which is supported by the excess air normally found in the gas turbine exhaust gases. Supplementary firing greatly increases the cost of construction of the steam generator, since it gives rise to problems due to radiant heat and requires materials of greater cost in order to withstand the heat. These disadvantages occur both in unpressurized or pressurized supplementary fired heat exchange units, although the pressurized units enable smaller heat exchange surfaces due to the better heat transfer coefficients obtained in the pressurized units.

It has previously been suggested that dual steam generators of the pressurized fired type might be supplied with combustion air by the compressor of a gas turbine, with the combustion air substantially equally divided between the two vapor generators. This arrangement is subject to the attendant expense of pressurized fired boilers as pointed out above.

It has also been suggested that saturated steam might be generated at more than one pressure level using exhaust from more than one gas turbine section, but here one again confronts the added expense of additional piping, steam drums, etc. for the steam at various pressures.

Accordingly, one object of the present invention is to provide an improved combined steam turbine gas turbine cycle which employs a simple unfired steam generator construction while receiving benefits of reduced cost due to pressurization of some of the heat exchange elements.

Patented June 20, 1967 Another object of the invention is to provide an improved combined steam turbine gas turbine cycle which makes possible more elficient and low cost steam generation in an unfired heat recovery steam generator, so as to decrease the required relative size of the gas turbine in a combined cycle.

Another object of the invention is to provide an improved heat recovery steam generator for generating and superheating steam in which the heat exchange elements are disposed so as to be compatible with the exhaust gas conditions from two series-connected gas turbine sections with refiring external to the heat recovery unit and between turbine sections.

The subject matter of the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of practice, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which:

FIG. 1 is a simplified schematic view of a combined steam turbine gas turbine cycle, and

FIG. 2 is a graph illustrating the heat distribution in the heat recovery units.

Briefly stated, the invention is practiced by dividing the steam genertaor into a pressurized unit and unpressurized unit. Dual steam evaporators for generating saturated steam and supplying it to a common drum are disposed in the two units. The pressurized unit is heated by exhaust gas from a high pressure gas turbine and the exhaust gas is then reheated in a second gas turbine combustor and, after passing through the low pressure gas turbine section, it passes through the unpressurized unit. Heat exchange elements for superheating and reheating the steam are placed in the pressurized unit ahead of one of the evaporators, While an economizer is placed in the unpressurized unit behind the other evaporator. In this manner, the two units become ideally matched to the exhaust gas conditions in the two section gas turbine unit.

Referring now to FIG. 1 of the drawing, a two-shaft gas turbine 1 is combined with a steam turbine 2 by generating steam for steam turbine 2 in an unfired heat recovery boiler 3. Gas turbine 1 comprises a compressor 4, a first combustor 5 for burning fuel, a high pressure gas turbine section 6, which serves to drive the compressor on one shaft, a second combustor 7 for burning additional fuel, and a low pressure gas turbine section 8 driving a load 9 such as a generator on a second shaft.

The steam turbine 2, illustrated by way of example as a tandem compound reheat machine, comprises a high pressure turbine section 10, a reheat section 11, and a low pressure section 12 driving a second load 13 such as a generator. Condensed steam from the low pressure unit 12 is heated by feedwater heaters 14, passes through deaerator 15 and is pumped by boiler feed pump 16 to steam generator 3 in the usual manner.

The heat recovery boiler 3 comprises a pressurized unit 17 and a non-pressurized unit 18 utilizing a common saturated steam drum 19. Dual evaporating heat exchange elements 2t), 21 are disposed in the pressurized section 17 and unpressurized section 18 respectively and are connected to receive feedwater from and to supply saturated steam to a common drum 19. The steam drum 19 is supplied With feedwater heated by passing it through an economizer 22 disposed in the lower temperature part of the unpressurized unit 18. Saturated steam from drum 19 is superheated in a super-heating element 23 disposed in a higher temperature portion of the pressurized unit 17, whence it flows to the high pressure steam turbine section 10. The steam may also be reheated in reheater 24 also disposedin the higher temperature portion of the pressurized heat exchange unit 17.

The operation of the improved cycle is as follows. The exhaust gases from high pressure gas turbine section 6 discharge at a pressure considerably above atmospheric and pass through pressurized heat unit 17 without substantial pressure drop. There the gases serve to superheat and reheat the steam as well as to generate about half of the saturated steam in the cycle. Since the heat exchange process in unit 17 takes place above atmospheric pressure, substantially improved heat transfer coeflicients prevail and the size and cost of heat exchange elements 20, 23, 24 are reduced accordingly.

Also, since the first stage of combustion takes place in combustor and the gas is cooled somewhat by passing through the high pressure gas turbine section 6, temperatures in the pressurized section 17 are at manageable values and do not require any special type of construction.

Exhaust gas from unit 17 goes to combustor 7 where additional fuel is burned. After the second stage of combustion takes place in combustor 7 and the gas is expanded through low pressure gas turbine section 8, it exhausts at substantially atmospheric pressure and passes through the unpressurized heat exchange unit 18. The remainder of the saturated steam is generated in the other evaporating heat exchanger 21 and the temperature is further reduced to the lowest practical value by means of economizer 22.

The construction is such that an ideal balancing can be achieved between the heat recovered in the pressurized and unpressurized sections 17, 18, at the exhaust condi tions of the gas turbine 1.

FIG. 2 is a graph illustrating this point, wherein the abscissa represents the percent of heat given up by the gas turbine cycle and recovered in the steam cycle, while the ordinate shows the temperature. The graph shows curves for the particular example discussed hereinafter. Line 17 represents heat lost by the gas turbine gases in the pressurized heatexchange unit 17, while line 18' represents heat lost by the gas turbine exhaust gases in heat exchange 18. Reheating in combustor 7 is shown by line 7 The dashed lines illustrate heat recovered in the steam cycle. Line 22' shows feedwater heating in the economizer 22; line portion 21 shows saturated steam genera tion in one of the dual evaporators 21; and line portion 20' shows steam generation in the other dual evaporator section. Line 23' shows superheating in superheater 23 and line 24 shows reheating in reheater 24. It will be observed from the graph that about one-half of the heat is recovered in each of the heat exchanger units 17, 18.

Line 25 on the graph is drawn to show a conventional waste heat recovery cycle wherein steam is generated by exhaust heat from a low pressure turbine alone. The slope of line 25 is determined by the gas temperature from the exhaust of low pressure turbine section 8 and the lowest permissible gas temperature at the outlet of the heat recovery unit to prevent corrosion of the tubes by condensation of harmful combustion products. It will be observed that line 25 illustrates the basic inability of the conventional exhaust heat cycle to generate high pressure superheated steam since the steam curve represented by the dashed lines would have to lie completely below line 25. However, by observing lines 17, 18', it will be appreciated that by dividing th heat exchange function into pressurized and unpressurized portions, the slopes ofthe curves can closely match the inherent slopes of the steam generation curve and give acceptable temperature differences in the various heat exchange sections while dividing the steam generation between the two evaporating sections.

By way of example, a cycle which provided the data for FIG. 2 consists of acycle wherein gas turbine 1 furnishes about one-third of the total power to load 9 while the steam turbine 2 furnishes the remaining two-thirds of 4 the total power to load 13. This is in contrast to the conventional type of unfired exhaust heat combined cycle, wherein the gas turbine has to be large in relation to the steam turbine, or a ratio of about 3 to 2 for gas vs. steam power.

In the exemplary cycle, exhaust conditions from the high pressure gas turbine are gas at 1200* F. at about three atmospheres absolute pressure. The gas temperature is reduced to 680 F. while passing through heat exchange unit 17 and is again increased in temperature in the second combustor 7 so that it exhausts from low pressure gas turbine section 8 at about 900 F. The stack exhaust temperature from unit 18 is 350 F. as illustrated in FIG. 2.

The above-described gas turbine heat recovery unit will supply steam at 1800 p.s.i.g./1000 F/with reheat to 1000 F. The station heat rate of the cycle after allowing for various auxiliaries is 9400 B.t.u./kw..hr., or about 5 percent better than a conventional steam cycle with the same steam conditions and output.

A two-shaft gas turbine is believed to be the best type to use because of maximum operating flexibility and efliciency at partial loads. Two shaft is meant to include not only a conventional industrial two-shaft gas turbine, but also arrangements which have been suggested for utilizing several jet engines in conjunction with a load turbine on a separate shaft, as in US Patent 3,169,366, for example. However, the invention is also applicable for single shaft gas turbines with means to extract gas from the high pressure section and to reintroduce it into the lower pressure turbine section.

It will be apparent to those skilled in the art that refinements might include the use of an intermediate pressure heat exchanger unit in addition to the two shown, which would employ a corresponding additional intermediate. pressure gas turbine section with an additional combustor.

One slight modification which will occur to those skilled in the art is to locate a portion of either superheater 23 or reheater 24 in the atmospheric pressure heat exchange unit 18. The. former would cause a portion of the superheating or reheating to occur under curve 18 of FIG. 2 and to shift a corresponding portion of the evaporating curve 21' to lie under curve 17' (high pressure heat exchange unit 17). This may be desirable in some cases to obtain a better arrangement of heat exchange surface, but would tend to lose some of the economic advantages of the preferred embodiment shown, wherein the more expensive superheating and reheating elements are all located in the more efiicient pressurized unit.

While it has been shown what is considered to be the preferred embodiment of the invention, other modifications will occur to those skilled in the art. It is, of course, intended to cover by the appended claims all such modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a combined cycle having a steam turbine utilizing steam generated in a heat recovery boiler by a gas turbine with high and low pressure turbine sections, the combination of:

a first high pressure unfired heat recovery unit connected to receive pressurized combustion gases from the high pressure turbine section of the gas turbine,

a second low pressure unfired heat recovery unit connected to receive exhaust gas from the low pressure turbine section of the gas turbine, and

first and second evaporating heat exchangers disposed in the first and second units respectively and connected to a common saturated steam collecting means,

2. The combination according to claim 1 including a third heat exchanger disposed .in a higher temperature portion of said first unit and connected to said common collecting means for superheating the steam, and also including a fourth heat exchanger disposed in a lower temperature portion of said second unit and connected to supply heated feedwater to both of the first and second heat exchangers.

3. The combination according to claim 1 including means for reheating the pressurized gas turbine combustion gases by burning additional fuel therein between the first unit and the low pressure turbine section.

4. A combined steam turbine gas turbine powerplant comprising:

a steam turbine having high pressure and reheat steam turbine sections,

a gas turbine having a compressor and a high pressure turbine section disposed on a first shaft and a low pressure turbine section disposed on a second shaft and including first and second combustors supplying combustion gases to the high pressure and low pressure turbine sections respectively,

a heat recovery boiler comprising a pressurized unit connected to receive pressurized exhaust gas from the high pressure turbine section and to supply gas therefrom to said second combustor, said heat recovery unit also including an unpressurized unit connected to receive exhaust gas from the low pressure turbine section and to discharge it to the atmosphere,

a common steam drum associated with both of said units,

first and second evaporating heat exchangers disposed in the pressurized and unpressurized units respectively and connected to receive heated feedwater from and to supply saturated steam to said common drurn,

an economizer disposed in a lower temperature portion of the unpressurized unit and connected to supply heated feedwater to said common drum, and

a superheater and a reheater disposed in a higher temperature portion of the pressurized unit receiving steam from the steam drum and connected to supply superheated steam and reheated steam to said steam turbine.

References Cited UNITED STATES PATENTS 2,702,026 2/1955 Dalin 1227 3,002,347 10/1961 Sprague 60-3918 X 3,147,742 9/1964 May 1227 CARLTON R. CROYLE, Primary Examiner. 

1. IN A COMBINED CYCLE HAVING A STEAM TURBINE UTILIZING STEAM GENERATED IN A HEAT RECOVERY BOILER BY A GAS TURBINE WITH HIGH AND LOW PRESSURE TURBINE SECTIONS, THE COMBINATION OF: A FIRST HIGH PRESSURE UNFIRED HEAT RECOVERY UNIT CONNECTED TO RECEIVE PRESSURIZED COMBUSTION GASES FROM THE HIGH PRESSURE TURBINE SECTION OF THE GAS TURBINE, 